Gs650e carb tuning for mac 4-1 and velocity stacks

[ddlaz]

Good evening all. Ive been tinkering with this carb issue for a few nights and still cant get this issue sorted out.
Bike is a gs650e with mac 4-1and short velocity stacks with a pantyhose for a filter.
I have read that it was going to be hard to tune but tinkerers cant help but tinker...
The bike ran great before i tore it down and modified it. Unfortunately it cant be reverted to stock, otherwise i would do it just to confirm that its a tuning issue.
I installed a dynojet kit and am having pause/stutter issues cruising at 5700ish rpm in 5th gear. The exhaust has a different, "bogging" tone when it does the stuttering. It will only stutter for a short second then return to normal and repeat until i either let off the throttle or open it more.
Im at about half throttle or so when this happens.

Things that ive tried:
Smaller main jet down to a dj146
Larger main jet up to dj160
Adjusted fuel level to be within spec, higher and lower than spec by 2mm
Lowered and raised needle clip from end grooves to middle at each main jet sizes
New plugs
Coil relay mod (had 10v at coils)


The issue is still there at all the different changes!
I did a plug chop during the stuttering and the plugs turned out a bit on the darker side but not sooty and black.

I have not checked timing as i dont have a timing gun...yet. but could ignition timing affect steady cruising?

Other than the cruising issue, it runs great!

*** Solved *** as Mark suggested(and others) these CV carbs just did not want to work without some vacuum that the velocity stacks couldn't provide.

 

[tom203]

A tinkerer with panty hose?? Hmm, I remember my youth differently but Ok, have you tried thicker panty hose or something more restrictive airflow wise to help vacuum diaphragms out (the studdering stuff). With these shorty velocity stacks, you might be stressing the dynojet idea.

 

[T3nrec]

At half throttle, wouldn't your needle be where you should be looking? I didn't notice anything about jet needle adjustments in your list of tuning. My 2 cents is play with the needle adjustment clips (or spacer thickness) and see if it changes.

 

[Guest]

I installed a dynojet kit and am having pause/stutter issues cruising at 5700ish rpm in 5th gear.

What happens if you pull the choke while it is stuttering?


Mark

 

[ddlaz]

A tinkerer with panty hose?? Hmm, I remember my youth differently but Ok, have you tried thicker panty hose or something more restrictive airflow wise to help vacuum diaphragms out (the studdering stuff). With these shorty velocity stacks, you might be stressing the dynojet idea.

I didnt go into more detail but i did try putting duct tape over part of the stacks to see if I was severely rich or something. No effect.

T3nrec - i did mention needle clip movements from one end to the other on all the different main jet sizes ive tried. No difference.

Pulling choke while stuttering makes it bog down even more but its more of a rich bog than the stuttering i feel.

 

[JJ]

You should have resisted the urge to tinker, especially when you don't know what you're doing.

Good luck!

Panty hose.... LMAO!!

 

[boris.h]

I calculated for these carbs size (bs32), that 1.25mm should be ok, so 1.25mm is similar to a mikuni # 112.5 one... far from those of dynojet... maybe your smaller main jet is too large?

 

[Agemax]

I calculated for these carbs size (bs32), that 1.25mm should be ok, so 1.25mm is similar to a mikuni # 112.5 one... far from those of dynojet... maybe your smaller main jet is too large?

Please explain how you "calculated" this, what formula did you use?

 

[boris.h]

ok, so I apologize for my last message. There are some formulas to get the "right" carb diameter. If I consider, next to that a 32mm venturi diameter (BS32), a first formula to get the main jet is 0.05xd.
So if I have 32mm, I need a 1.60mm main jet. 1.60mm is the same like a dynojet #148 so I think that dynojet is right about that... and according to standard charts, #148 dynojet = mikuni #135 or # 137.5....

 

[ddlaz]

You should have resisted the urge to tinker, especially when you don't know what you're doing.

Good luck!

Panty hose.... LMAO!!

What exactly is it that i dont know how to do ? How to solve a hiccup through the rpm range? Who could predict that? That was such a negative, non-value added comment.

Panty hose are actually used often on itb setups on cars. The stacks came with brass screens but it just wasnt fine enough for my liking.

 

[ddlaz]

I calculated for these carbs size (bs32), that 1.25mm should be ok, so 1.25mm is similar to a mikuni # 112.5 one... far from those of dynojet... maybe your smaller main jet is too large?

I considered that but when i run the 146, i get white plugs on a plug chop so it is actually too lean.

 

[BoCoMoMann]

Put the stock airbox back on and it will run better than anything you can do with stacks - except at full throttle and high RPM.

 

[T3nrec]

T3nrec - i did mention needle clip movements from one end to the other on all the different main jet sizes ive tried. No difference

My bad, missed it first time around

 

[Nessism]

Seems like a lot of effort and expense for a mod that adds zero functional benefit and a LOT of negative effect. No doubt this bike is some bobber/rat rod/cafe jobby.

 

[tkent02]

Not much helpful advice in this thread yet, so I'll try to help. I have the same engine, at least the same top end and carburetors on my 550, and I also cannot use a stock airbox. The 550 airbox won't fit the CV carbs which are positioned farther back than they are on the 550 head, and the 650 airbox won't fit in the 550 frame at all. I have K&N pods, the double ones since the singles also don't fit, a Kerker pipe and a Dynojet kit, haven't had the time to get it tweaked to perfection yet.

Can you please start with as detailed a description as you can about the problem? I read the first post a few times, not quite getting it. Is it only RPM related? Throttle positon related? Only happen in fifth gear?

Just a quick thought, if those jetting changes you have done have no effect, maybe it's the ignition?

 

[boris.h]

I will try 1.50mm to start, so 1.50mm is about DJ# 140.. I have the same configurationthan you, velocity stacks, bandit header whitout any muffler for the instant
How is your idle? Did you screw the idle screw a lot to get a good idle? Mine is almost completely unscrewed to have a 900 rpm idle...

 

[boris.h]

ok, lot of hesitation again, and the engine heats a lot. White plugs when cruising and accelerating. Going to drill my jets to 1.65mm --> DJ152 mains.... will test it by the end of the week

 

[ddlaz]

Not much helpful advice in this thread yet, so I'll try to help. I have the same engine, at least the same top end and carburetors on my 550, and I also cannot use a stock airbox. The 550 airbox won't fit the CV carbs which are positioned farther back than they are on the 550 head, and the 650 airbox won't fit in the 550 frame at all. I have K&N pods, the double ones since the singles also don't fit, a Kerker pipe and a Dynojet kit, haven't had the time to get it tweaked to perfection yet.

Can you please start with as detailed a description as you can about the problem? I read the first post a few times, not quite getting it. Is it only RPM related? Throttle positon related? Only happen in fifth gear?

Just a quick thought, if those jetting changes you have done have no effect, maybe it's the ignition?

The problem is hesitation or surging or bogging, however you want to call it, while cruising in 5th gear on the highway around 5700 rpm. It's not at a specific frequency but it will continue bogging then clear then bog then clear constantly until I change throttle positions to either speed up or slow down.
It is more prominent in 5th but that's because I don't do much cruising at 5700 unless I'm on the highway.
Yes I did mention in my post that I'm thinking it's an ignition issue.
Since my OP I've checked the timing and the advancing is working properly as it will advance to 40 degrees but there is an intermittent flicker that throws off the timing. Not sure how to explain it other than its not a constant timing light flash.

 

[Guest]

The problem is hesitation or surging or bogging, however you want to call it, while cruising in 5th gear on the highway around 5700 rpm. It's not at a specific frequency but it will continue bogging then clear then bog then clear constantly until I change throttle positions to either speed up or slow down.

As tom203 mentioned, CV carbs need some intake vacuum to operate well. This may be vacuum related and something that simply can't be cleared while running the velocity stacks. Does 5700rpm equate to a speed you run regularly? If you really want to keep the stacks would changing gearing a tooth or two on the back solve your issue? Is it possible to try longer velocity (or shorter) stacks to see if that changes your issue? How does it run if you remove the stacks altogether?


Mark

 

[JJ]

Velocity stacks belong on the race track, or in the garbage, NOT on the street...